Field of the Invention
The present invention relates to an absolute encoder.
Description of the Related Art
Conventionally, an absolute encoder is used to measure the stage position of an apparatus. An absolute linear encoder records, on a scale, a pattern made to correspond to codes of 1s and 0s, and optically or electromagnetically detects the pattern. Japanese Patent Laid-Open No. 2012-037392 discloses an absolute encoder adopting a method of arraying, in the moving direction of a scale, a grating whose reflectance changes in correspondence with a code of 1 or 0.
Since the reflectance may change due to the influence of a defect in the scale (dust on the scale, a flaw in the grating, or the like) to cause erroneous code detection, there is known a method of detecting such error and outputting a warning.
In Japanese Patent Laid-Open No. 03-274414, a track is formed by a one-track absolute pattern such that m codes Xi (i=1, . . . , m) detected by m (m is 2 or more) sensors satisfy an equation f(X1, . . . , Xm)=constant C (f: function). The numerical solution of the equation f is obtained by using the outputs of the sensors, and compared with the constant C. If these values are different from each other, a warning signal is output.
In the method described in Japanese Patent Laid-Open No. 03-274414, it is impossible to acquire correct absolute position information during a period in which a detector passes through an error factor portion. If the error factor portion falls outside a detection region, correct position information is output, as a matter of course. Conventionally, during the period, position information obtained by performing estimation based on speed information and the like is output at best.